1. Field of the Invention
Novel perfluoro cycloalkane, perfluorinated aromatic compounds and perfluorotrialkyl amine solvents for dissolving fluorinated polymers having carboxyl groups and an equivalent weight greater than 900 are disclosed. The solvents have a critical temperature greater than 150.degree. C. Also disclosed are solutions of the polymers in the solvents, and a process for forming the polymer solution.
2. Technical Background
This invention concerns novel solvents for dissolving certain alkyl carboxylate forms of fluorinated polymers which are copolymers of tetrafluoroethylene ("TFE") and certain perfluorovinylether comonomers. The solvents readily dissolve such polymers more completely and at lower temperatures than previously disclosed solvents, an advantage in equipment utilization and other manufacturing concerns. Solutions of the polymers are particularly useful for preparing ion exchange membranes, including membranes for use in an electrolytic cell such as a chloralkali cell or a fuel cell.
The prior art generally discloses the solubility of certain fluorinated polymers but there is no mention of full solutions of carboxylic fluorinated polymers having high equivalent weights in this class of inventive solvents, particularly at low temperatures.
Some solvents are known in the prior art for this class of carboxylic fluorinated polymers; however, some of the solvents are very expensive or require high temperature to dissolve the polymer. The prior art teaches solvents for carboxylic fluorinated polymers with equivalent weights below about 900 and solvents for such polymers which require elevated temperatures or pressures. There is no teaching of a solvent for the ester form of carboxylic fluorinated polymers with an equivalent weight greater than 900 at low temperatures.
For example, U.S. Pat. Nos. 4,650,551; 4,778,723 and 4,784,900 disclose solutions near room temperature of highly swollen dispersions of resins which are copolymers of TFE and perfluorovinylether comonomers with acidic end groups having low equivalent weights less than 850. Heavily fluorinated alkanes, like 1,2-dibromotetrafluoroethane and 1,1,2-trichlorotrifluoroethane were used as "solvents". Examples of room temperature solutions were also given using polyhalogenated alkyl ethers with boiling points less than 190.degree. C.; the equivalent weight for these resins was exceedingly low at 690 or lower.
Fluorinated polymer resins commercially known as Nafion.RTM. (available from E. I. du Pont de Nemours and Company) with an equivalent weight greater than about 1000, must be dissolved at elevated temperatures and pressures due to the presence of crystallinity. U.S. Pat. Nos. 4,266,036; 4,272,560 and 4,298,697 discuss the use of high boiling perhalogenated alkyl ethers and perhalogenated alkanes as solvents at elevated temperatures. The examples describe the dissolution of 1050 to 1200 equivalent weight resins in oligomers of chlorotrifluoroethylene ("CTFE") at temperatures of about 225.degree. to 250.degree. C.; however, the polymers are more likely to decarboxylate at high temperatures. Chemical degradation of the polymer may occur in solutions of CTFE oligomer at 200.degree. C.
U.S. Pat. No. 4,535,112 describes dispersions or partial solutions of 1050 to 1200 equivalent weight polymers in a wide variety of heavily fluorinated liquids for purposes of repairing cation exchange membranes. These liquids include perfluorodecanoic acid, perfluorotributylamine, pentafluorophenol, pentafluorobenzoic acid, perfluoro-1-methyldecalin and decafluorobiphenyl. There is no mention of full solutions of carboxylic fluorinated polymers having high equivalent weights at low temperatures.
An article entitled "Solubility Characteristics of Perfluorinated Polymers with Sulfonyl Fluoride Functunality" by McCain and Covitch discusses the solubility of so-called sulfonyl fluorinated polymers, but does not mention alkyl carboxylate forms of fluorinated polymers.